Monolithic PN junction isolated integrated circuit (IC) devices employ a common semiconductor substrate. A plurality of such devices is fabricated simultaneously on a semiconductor substrate wafer. One of the process steps involves the diffusion of impurities completely through an epitaxially deposited semiconductor layer. This is called the isolation diffusion because it produces a plurality of isolated regions or tubs of epitaxial material. Circuit element devices such as transistors, diodes, or resistors are then created in the tubs. These circuit elements are ultimately interconnected by metallization applied over a passivating oxide layer that is created over the wafer surface.
The isolation diffusion must effectively penetrate the epitaxial layer. It must be deep enough to fully penetrate the epitaxial layer even at its thickest extremes on the wafer. Typically the process is operated so that substantial over-diffusion occurs. Then the process variables will not result in incomplete penetration. However, this concept is often carried to an extreme wherein greatly excessive isolation penetration is achieved. Since the diffusion penetration is accompanied by a lateral surface extension excessive diffusion will reduce the wafer surface area that could be used for circuit element devices, or it may result in electrical short circuits.
It would be desirable to monitor the isolation diffusion process to make sure that the isolation is effective and that the isolation over-diffusion is not excessive.